The wormers make the horse pass through all worms, so if you worm them and then put them out there is going to be worms all over the field which kind of defeats the objective. Thats what ive been told anyway.
Depends entirely on what you are worming for and what you are worming with. Some wormers will kill only mature forms, some will kill eggs and some wont. If you dont use a wormer that kills eggs then you are simply expelling them back onto your pasture.
Some wormers continue to expell for days or even weeks after that actual dosing so you cant really generalise on this.